It is known to produce a semipermeable membrane from a cellulose acetate. The cellulose acetate semipermeable membrane is prepared in such a manner that: a dope (solution) is prepared by dissolving a cellulose acetate in a mixed solvent consisting of acetone and formamide or a mixture of a metal salt such as Mg(ClO.sub.4).sub.2, water and acetone; a thin layer of the dope is formed on a base surface; a portion of the mixed solvent is evaporated from a surface of the thin layer of the dope, and; then, the thin layer of the dope is immersed in cold water to coagulate it and form a semipermeable membrane. However, the conventional semipermeable membrane consisting of the cellulose acetate is unsatisfactory in its heat-resisting property, resistance to chemicals, anti-bacterial property, resistances to acid and base, resistance to compression, and resistance to chlorine.
In order to obtain a semipermeable membrane having a satisfactory heat-resisting property, chemical-resisting property and compression-resisting property, various attempts have been made to prepare a semipermeable membrane from various aromatic polyamides and aromatic polyimides. In this regard, it was expected that semipermeable membranes having excellent heat-resisting and chemical-resisting properties, and mechanical properties, could be provided from aromatic polyimide type materials. However, an aromatic polyimide semipermeable membrane could be prepared only by very complicated processes that is, in one conventional process, the aromatic polyimide semipermeable membrane can only be produced in such a complicated manner that: a solution of a polyamic acid which has been produced by a polymerization reaction of an aromatic tetracarboxylic dianhydride with an aromatic diamine is prepared; a thin layer is formed from the solution; the thin layer of the solution is coagulated by using a coagulating liquid to provide a polyamic acid membrane, and; then, the polyamic acid membrane is imidized to provide a polyimide semipermeable membrane. In another conventional process, the aromatic polyimide semipermeable membrane can only be produced in such a complicated manner that: a thin layer of a solution of a polyamic acid is formed while partially imidizing the polyamic acid; the thin layer of the partially imidized polyamic acid solution is coagulated with a coagulating liquid, and; finally, the coagulated partially imidized polyamic acid membrane is completely imidized. In still another conventional process, the aromatic polyimide semipermeable membrane can only be produced in such a complicated manner that: a thin layer of a polyamic acid solution is coagulated with a coagulating liquid containing an imidizing agent while imidizing the polyamic acid, and; then, the coagulated polyimide membrane is heated to provide a polyimide semipermeable membrane.
As is clear from the above explanations, in the conventional processes, it is necessary to convert the polyamic acid to an aromatic polyimide by imidizing it during or after preparing the thin layer of the polyamic acid solution, and/or during or after coagulating the thin layer of the polyamic acid solution. Therefore, it is very difficult to precisely control the thin layer-forming, imidizing and coagulating procedures, so as to obtain products having a uniform and satisfactory semipermeable property. Furthermore, the aromatic polyimide semipermeable membranes produced by the conventional processes exhibit an unsatisfactory water-permeability and/or salt-rejection.
British Pat. No. 1,435,151 discloses a process for producing a semipermeable polyimide membrane in such a manner that: a thin layer of a solution of a polyamic acid, which has been produced by a polymerization reaction of an aromatic tetracarboxylic dianhydride with an aromatic diamine, is formed, and; the thin layer is coagulated with a coagulating liquid while imidizing the polyamic acid to convert it into an aromatic polyimide.
Furthermore, it is known that the dope (solution) of the polyamic acid is unstable and cannot be stored for a long period of time. Therefore, it is desired to provide a new process for producing the polyimide membrane without using the unstable polyamic acid solution.
U.S. Pat. No. 3,822,202 and U.S. Pat. No. 3,899,309 disclose another process for producing a gas-separating polyimide membrane. In this process: a specific aromatic polyimide, which contains a backbone chain and at least one rigid divalent sub-unit having two single bonds which extend from the sub-unit to the backbone chain and which are not co-linear to each other, the rigid divalent sub-unit being not capable of rotating at an angle of 360 degrees around at least one of the single bonds, and at least 50% of the atoms in the backbone chain being located in aromatic radicals in the backbone chain, is dissolved in an organic solvent such as dimethyl acetamide; the solution is spread to form a thin layer, and; the thin layer of the solution is dried to form a gas-separating membrane.
However, the gas-separating property of the polyimide membranes produced by any of the above-mentioned known processes is unsatisfactory.